1. Field of the Invention
The present invention relates to methods and techniques for converting agrowaste biomass which are solid lignocellulosic materials to liquid biofuels with an energy content that is more than 70% of that of the solid agrowastes themselves. More particularly, this invention relates to methods and techniques which in toto comprise the process of achieving this conversion by means of a set of sequential and concurrent processing steps requiring temperatures of between 275 K to 500 K, pressures between 0.1 MPa to 1 MPa and total energy inputs ca. 8 GJ per dry ton of solid agrowaste biomass. Even more particularly, this invention relates to the judicious selection, arrangement and coordinated operation of processing steps, and their constituent unit operations, that result initially in a separation of the solid lignocellulosic materials into fractions consisting essentially of each of the chemical components of such agrowastes viz. 35-50% cellulose, 15-35% hemicellulose, 10-30% lignin, 2-5% extractives and 2-3% ash and subsequently in reformation of the fractionated solid agrowastes and reconstitution to form a liquid biofuel.
2. Description of Prior Art
Processes for converting solid lignocellulosic materials to liquid biofuels have been known and practiced for a relatively long period of time in human history eg the ancient Egyptians carried out wood distillation to produce charcoal, tar and pyroligneous oils.
Wood distillation, wherein the central processing step is pyrolysis which is the technique of applying high heat, ca. 800 K, to lignocellulosic materials in the absence of air, to produce charcoal was a major industry during the 1800s, supplying the fuel for the industrial revolution, until it was replaced by coal. In the late 19th Century and early 20th Century wood distillation with subsequent collection of the condensable off-gases as pyroligneous oils was still profitable for producing soluble tar, pitch, creosote oil, chemicals. The wood distillation industry declined in the 1930s due to the advent of the petrochemical industry. Modern embodiments of this process yield a liquid biofuel product in 60-75% yield containing ca. 35% of the energy contained in the feed material. (cf D. Mohan et al: Energy & Fuels, 2006, 20, 848-889.)
This traditional method was supplemented by a synthetic process developed ca. 1930 which utilizes a pyrolysis reactor operating as a gasifier, ca. 1200 K, by injecting substoichiometric oxygen into the reactor core to partially burn the biomass to ash and reducing gases. After purification the syngas, hydrogen and carbon monoxide in a 2 to 1 ratio, is transformed by catalysts under high pressure and heat, to form methanol. This method produces ca. 100 gallons of methanol per dry ton of feed material containing about 35% of the energy contained in the feed material. Modern variants of this method convert the syngas to other liquid biofuels such as ethanol or hydrocarbons containing ca. 40% of the energy contained in the feed material. (cf. A. P. C. Faaij et al: Biomass & Bioenergy, 2002, 23, 129-152). Another process known since the early 1800s and practiced on a comparatively small scale since the early 1900s prepared ethanol from wood by a sequential series of processing steps (i) Acid Catalyzed Wood Hydrolysis, at around 450 K (ii) Microbial Fermentation and (iii) Distillation (cf E. Boullanger: Distillerie Agricole et Industrielle; Paris: Ballière, 1924). The yield of ethanol was limited to ca. 100 L per dry ton containing ca. 10% of the energy contained in the feed material. Modern embodiments of this process for converting solid lignocellulosic materials to ethanol, particularly involving improvements in the operations of the first step by conducting it in two stages with the second stage being an enzymatic hydrolysis of the cellulose component (cf. USP #33972775 dt. 1976), have enabled an increase in the yield of ethanol to ca. 90 gallons of ethanol containing ca. 40% of the energy contained in the feed material. (cf. P. Zhang: J. Ind Microbiol Biotechnol, 2008, 35, 367-375; C. A. Cardonna et al: Bioresource Tech. 2007, 98, 2415-2497; T. W. Jeffries: Appl. Microbiol. Biotech. 2003, 63, 258-266).
Thus the methods known and practiced in the art to date are limited in the yield of energy contained in the liquid biofuels that are produced from lignocellulosic feed materials, generally in the range 35%-45%.